Generates a formula to scramble a Rubik's cube

scrambler.rb

#!/usr/bin/env ruby

module Scrambler
  FACES = %w[B L D R U F].freeze

  def self.run(steps)
    directions = []
    faces = []

    steps.times do
      source = FACES - last_and_parallel(faces.last)

      faces.push(source[rand(source.size)])
      directions.push(["'", nil][rand(2)])
    end

    faces.zip(directions)
  end

  def self.last_and_parallel(value)
    pairs = [%w[B F], %w[D U], %w[L R]]

    pairs.reduce { |memo, pair| pair.include?(value) ? pair : memo }
  end
end

scramble = Scrambler.run(20)

print scramble.map { |e| e.compact.join }.join(' ') + "\n"